linux-kernel - Re: [dm-devel] [PATCH 0/6] dax poison recovery with RWF_RECOVERY

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Date:   Thu, 4 Nov 2021 10:46:41 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jane Chu <jane.chu@...cle.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        "david@...morbit.com" <david@...morbit.com>,
        "vishal.l.verma@...el.com" <vishal.l.verma@...el.com>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "agk@...hat.com" <agk@...hat.com>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "ira.weiny@...el.com" <ira.weiny@...el.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "vgoyal@...hat.com" <vgoyal@...hat.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>
Subject: Re: [dm-devel] [PATCH 0/6] dax poison recovery with
 RWF_RECOVERY_DATA flag

On Thu, Nov 04, 2021 at 09:08:41AM -0700, Dan Williams wrote:
> Yes, atomic clear+write new data. The ability to atomic clear requires
> either a CPU with the ability to overwrite cachelines without doing a
> RMW cycle (MOVDIR64B), or it requires a device with a suitable
> slow-path mailbox command like the one defined for CXL devices (see
> section 8.2.9.5.4.3 Clear Poison in CXL 2.0).
> 
> I don't know why you think these devices don't perform wear-leveling
> with spare blocks?

Because the interface looks so broken.  But yes, apparently it's not
the media management that is broken but just the inteface that fakes
up byte level access.

> All kernel accesses do use it. They either route to
> pmem_copy_to_iter(), or like dm-writecache, call it directly. Do you
> see a kernel path that does not use that helper?

No, sorry.  My knowledge is out of date.
(nova does, but it is out of tree, and the lack of using
copy_mc_to_kernel is the least of its problems)